1. Field of the Invention
The present invention relates to a method for cleaning a semiconductor manufacturing apparatus, and more particularly, to prevention of corrosion during cleaning and residual gas after cleaning.
2. Description of the Related Art
In a cleaning method of a large-scaled reaction chamber used for a semiconductor manufacturing apparatus such as a batch-type low pressure chemical vapor deposition (LPCVD) apparatus, an etching method wherein a thermal reaction of a reactive gas such as chlorine trifluoride (ClF3) or nitrogen trifluoride (NF3) is used. A ClF3 gas and an NF3 gas, which are used in the cleaning, are very expensive. Moreover, a perfluoro compound (PFC) gas such as NF3 has serious effects on the environment, including global warming and the like. From the viewpoint of gas cost reduction and PFC reduction, a fluorine (F2) gas and a mixed gas including the F2 gas, are considered as alternatives (refer to Japanese Patent Laid-Open No. 2002-100618 (pp 5–9, FIG. 1)).
However, the F2 gas has extremely high reactivity and causes an inner wall surface of stainless steel to corrode and deteriorate due to a fluorination reaction. The stainless steel is used as a material for piping components such as conduits and valves provided at an outlet side of a reaction chamber. Moreover, the F2 gas reacts with water (H2O), which is adsorbed in inner walls of the reaction chamber and the conduits, to generate a hydrogen fluoride (HF) gas. Metal fluorides such as iron fluoride (FeF2) and chromium fluoride (CrF2) which are generated on the inner wall surface of the outlet-side piping component due to the fluorination reaction, store the hydrogen fluoride (HF) gas by coordinate bond. The HF gas stored on the inner wall surface of the outlet-side piping component is released during an LPCVD step, and back diffusion of the HF gas into the reaction chamber occurs. Thus fluorine contamination results in an interface between a semiconductor substrate and a deposited film or in the deposited film. In order to suppress the fluorine contamination, it is effective to cool the outlet-side piping component. By maintaining the inner wall surface of the outlet-side piping component at a low temperature, the corrosion of the stainless steel due to the fluorination reaction with the F2 gas is suppressed. However, when a silicon (Si) film and a silicon nitride (Si3N4) film deposited inside the reaction chamber are cleaned off by the F2 gas, a silicon tetrafluoride (SiF4) gas is generated as a reaction product. The SiF4 gas reacts with the HF gas to generate hexafluorosilicic acid (H2SiF6) which has a low vapor pressure. The low vapor pressure H2SiF6 is easily adsorbed into the cooled inner wall of the outlet-side piping component, thus becoming a fluorine contamination source.
For protection of the inner wall surface of the outlet-side piping component, corrosion-resistant surface coating is effective, such as alumina coating, nickel plating, polytetrafluoroethylene coating and the like. The stainless steel of the inner wall subjected to the surface coating can suppress the fluorination reaction even when heated. However, to apply the surface coating to everything, right down to a small sealing member on the outlet-side piping components, leads to an increase in costs. Thus, the versatility of the outlet-side piping component is limited. Moreover, some of the outlet-side piping components include a movable part such as bellows of a valve. Thus, damage caused by abrasion is unpreventable. Because of regular recoating, disassembly maintenance is required. As a result, the inoperative period of the semiconductor manufacturing apparatus is increased and thus the production capacity thereof is lowered.